This invention relates to light emitting diodes (LEDs).
Semiconductor LEDs emitting red, green, blue, infrared and ultraviolet light have been marketed for many years. For light generation in LEDs, radiative recombination of electrons and holes in an active layer is used. The active layer can be a usual p-n junction, heterojunction, single quantum well or multiple quantum wells. In some LEDs, a single quantum well or multiple quantum wells is or are used as the active layer. For fabrication of a highly efficient device, the number of carriers recombined inside the active layer should be maximized and the number of carriers recombined outside the active layer should be minimized. This needs optimization of capture rates for electrons and holes into the active layer. Usually in semiconductors, effective masses of holes are much higher and mobilities much less than those for electrons. Therefore, some of the electrons not captured in the active layer escape the active layer and recombine outside it. This prevents fabrication of highly efficient LED devices.
This invention provides an LED design based on a two well system with asynnnetric tunnelling, the system comprising first and second coupled wells, namely a wide well (WW) and an active quantum well (QW), the wells being coupled via a resonance tunnelling barrier (RTB) transparent for electrons and blocking for holes. Both the wide well and the active quantum well can be made of either a single quantum well (SQW) structure or a multiple quantum well (MQW) structure.
This invention allows enhancement of the number of electrons captured in the active layer with the quantum well and the fabrication of highly efficient LEDs.
The LED could be a GaN LED, a GaAs LED, an AlGaInP or a ZnSe LED for example. More generally, the LED could be one which is based on a III-V or II-VI group semiconductor.
A p-layer could be one of GaN, p-Al1xe2x88x92xN and p-type polycrystalline Gan example. More generally, the LED could be one which is based on a III-V or II-VI group semiconductor.
A substrate of the LED could be removable or have been removed by wet etching or laser ablation.
Ohmic contacts could be on the top and bottom or both on the top of the LED structure.
Resonance tunnelling between two wells means that the energy position of the WW bottom has to be equal to the energy position of the sub-band minimum of the active QW. This problem may be solved by adjusting the alloy composition in the WW and the QW, and choosing a proper QW width.
This design is based on the mass asymmetry of electrons and holes in GaN for example.
It allows the electron sub-band position in the active QW to be fitted to the bottom of the WW and at the same time the hole sub-band minimum in the QW to be kept lower than the bottom of the WW for holes and thus forbids hole penetration without thermal activation in the WW. It is important that even the small amount of thermally activated holes have no possibility to tunnel into the WW for the chosen barrier width because of their heavy mass.
Advantages
The structures to be described allow:
an increase of the capture efficiency of electrons into the active QW due to direct tunnelling of electrons from the WW into the QW
suppression of electron leakage into the p-type layer
elimination of parasitic light generated outside the active layer
the use of the WW as a good current spreading layer means an improvement of the quality of the active QW because of mismatch reduction (xcx9c4 times). The reason is that active In0.2Ga0.8N (for example) QW growth on a highly tensile strained thin GaN barrier has a lattice parameter close that of an In0.15Ga0.85N (for example) WW
the WW also works as a stopping layer for threading dislocations, because of the high stress on an interface of n-GaN/In0.15Ga0.85N (for example)
there is no need to use electron blocking layers as an element in this technology
reduction of the growth time, since we expect that the same quality of the active QW will be achieved for a thinner n-GaN layer for example in a GaN LED.
Finally, the structures should allow the manufacture of cheaper and more efficient LEDs than conventional ones.